The invention relates to a method of sealing a plastic lid to a plastic container and the combination produced thereby.
Many plastic containers are made of multiple layers of different plastics in order to achieve the desired barrier properties. For example, tubs, cups, jars and pails (hereinafter termed containers) are made of starting materials which are multi-layered laminates of a plastic which serves as a water or solvent barrier, and a second plastic which serves as an oxygen or air barrier. These barrier layers are usually held together with an adhesive or tie layer which facilitates the formation of the layers into a single sheet or billet, from which the container is made. The typical moisture or solvent barrier plastic is a type of polyolefin such as polypropylene or polyethylene and the typical oxygen barrier plastic is a polyvinylalcohol, a polyvinylidene chloride, a nylon, a polyacrylonitril or a polyester. The tie layer is usually made of the polyolefin onto which are grafted some polar monomers, typically containing an acid or anhydride moiety, like polypropylene with maleic anhydride grafted thereto.
The containers are usually made from multilayer sheet or billets by thermoforming or solid phase forming into the desired shape.
The making of these laminates is well known. If two thermoplastics are compatible, they can be laminated by melting at least the surfaces of the pieces in contact with one another and by applying pressure. If the two thermoplastics are not compatible, they can be made into a laminate by placing the tie layer between the incompatible layers.
Once the container has been formed and filled, a lid is fixed to the lip of the container. The lid should be made of a material which affords at least the same barrier properties as the container. The bond between the lid and the container should preferably afford a strong barrier to oxygen and water, yet should be weak enough to be readily removed when the user seeks access to the contents of the container.
One way of solving this sealing problem is to use aluminum lids which are heat-sealed to the container, wherein the lid is provided with an easily opened flap. For example, the aluminum lid may be melted or adhesively bonded to the container and provided with a spiral tear-away tab. A similar solution can be used with a plastic lid, i.e. the plastic lid may be melted to the lip of the container and the lid opened by cutting o tearing through the portion of the lid not bonded to the container. Neither of these solutions yields a reusable lid and often the container is supplied with two lids, one that is destructively removed and a second which is used to reseal the container (such as those currently used o coffee or peanut containers).
Typical methods of bonding a lid to a multilayer plastic container involves the use of an adhesive, spin welding, conventional convection/conductive heating and dielectric heating.
When dielectric heating is used to seal a lid on a container where one or both of the materials of the lid or container were polyolefins, the polyolefins were treated, either on their surfaces or by inclusion of additives in order to get the cup lip and the lid lip to heat to radio frequency or microwave frequencies (0.1-10,000 MHz). Neat polyolefins are essentially transparent to radio frequency or microwave radiation and will not adequately heat. Polymers containing polar moieties, such as vinyl alcohol or vinylidene chloride or copolymers of ethylene and CO will heat when subjected to microwave and/or radio frequency radiation.
A dielectrically heatable thermoplastic is one that absorbs enough power at a frequency between about 0.1 and about 300 MHz, preferably between about 0.1 and about 200 MHz, to melt, at a field strength lower than the arc strength, to melt. Such thermoplastics are generally those with Loss Indexes above about 0.08, preferably above 0.09 and most preferably above 0.1. (Dielectrically non-heatable thermoplastics are those which will not melt at the applied field and frequency. Generally, these thermoplastics are those with Loss Indexes below about 0.07, preferably below about 0.06 and most preferably below 0.05 at the applied field and frequency).
It is desirable that the barrier layers of a combination lid and container entirely surround the contents in order to protect them. It is therefore desirable that when multi-layered polar and non-polar plastics are used in the lip and the container that there be a bond between at least one of the non-polar and one of the polar layers when the lid is sealed to the container. On the other hand, it is desirable that the lid be easily removable without damage to the lid or the container. It is also desirable that the lid be reusable, i.e. that it be such that it can be repeatedly fixed to the container, not bonded to the container but have the capability of snapping onto the container.
The conventional bond and the conventional bonding method used between multilayer plastic lids and containers rely on a compromise. It is conceded that there will be a breach in at least one and probably two of the layers. That is, conventional layered lids and containers are made so that the oxygen barrier layer or layers have at least one moisture barrier layer between the oxygen barrier layer or layers and the environment, and between the oxygen barrier layer or layers and the contents. But, at the weld between the lid and container typically the oxygen barrier layer of the lid and container is not continuous and often one or more of the moisture barrier layers is not continuous. However, typically the thickness of the package at the weld line between the lid and container is considerably more, as much as ten times more, for example, than is the thickness of the body of the container. This increased thickness partially compensates for the break or breaks in the oxygen barrier layer or layers. That is, in a polypropylene, polyvinyl alcohol, polypropylene layered package, polypropylene is not considered a particularly good oxygen barrier when present in a ten (10) mil thickness, as it might be present in the body of a cup. But, when the thickness at the weld line is one hundred (100) mils, it partially compensates for the break in the polyvinyl alcohol which occurs at the weld line between the cup and the lid.
Even so, the risk of food spoilage and/or the occurance of botulism precludes extensive unrefrigerated shelf lifes for containers having breaks in the oxygen barrier. A container and lid assembly system need be developed that results in an unbroken oxygen barrier layer, yet allows the lid to be removed and subsequently placed back on to the container.